A present invention relates to a process for manufacturing uranium oxide powder from UF.sub.6 using a dry conversion, which is low in fluorine contaent and high in activity.
As conventional processes which manufacture uranium oxide from UF.sub.6 as a starting material by gas phase reaction, there are known two processes, that is, a process which reacts UF.sub.6 with steam and gydrogen gas at high temperatures and another process which manufactures uranium oxide from UF.sub.6 in the presence of the flame ignited by hydrogen gas and oxygen gas. The former is described in Japanese Patent Publication No. 18658 of 1961, Japanese Patent Laid Ipen No. 92124 of 1981 (U.S. Pat. No. 4397824), and the latter is described in Japanese Patent Publication No. 10095 of 1966, Japanese Patent Publication No. 24998 of 1976 (U.S. Pat. No. 3796672 Japanese Patent Publication No. 16976 of 1980. These processes are called a dry conversion process because of their manufacturing uranium oxide by gas phase reaction, while another process which comprises hydrolyzing UF.sub.6 to UO.sub.2 F.sub.2 solution, adding ammonia or ammonia and CO.sub.2 gases to the UO.sub.2 F.sub.2 solution to form ADU or AUC and manufacturing uranium oxide from ADU or AUC is called a wet conversion process.
The conversion process which reacts UF.sub.6 with steam and hydrogen gas at high temperature is mainly due to the following reaction machanism. ##STR1## But, it is known that these reactions are accompanied by many side reactions at the same time and that UF.sub.4 is formed partly. Therefore, it is also known that the uranium dioxide powder obtained by the gas phase reaction is relatively high in fluorine content. Further, these reactors must be heated up to high temperatures because UF.sub.6 reacts with steam and hydrogen gas at the high temperatures.
On the other hand, the process which manufactures uranium oxide powder from UF.sub.6 in the presence of the flame ignited by hydrogen gas and oxygen gas is mainly due to the following reaction mechanism. ##STR2## In this reaction, when the ratio of oxygen to hydrogen is more excessive, U.sub.3 O.sub.8 rich composotion is formed. This gas phase reaction is required to maintain the hydrogen flame of the range of 600.degree. to 900.degree. C. Therefore, a considerably excessive amount of hydrogen gas, in addition to the gas volume required for the conversion from UF.sub.6 to UO.sub.2, is required and temperature of 600.degree.-900.degree. C. can be maintained by combustion of the excessive hydrogen gas.
The reason why the temperature above 600.degree. C. is required in this reaction is thought to be due to a slow rate of the reaction of UF.sub.6 with hydrogen and requirement of a considerable amount of activating energy. Further, it is known that the uranium oxide obtained by this gas phase reaction is higher in fluorine content than the uranium oxide obtained by the conventional wet conversion process.
As the uranium oxide powder obtained by the conventional dry conversion process is relatively high in fluorine content, it has a undesirable effect for manufacturing uranium dioxide pellet.
In the process that UF.sub.6 reacts with steam and hydrogen at high temperatures, the reactor is required to be heated internally. In the process which manufactures uranium oxide powder in the presence of the flame ignited by hydrogen and oxygen, considerably excessive hydrogen gas is required for UF.sub.6 to maintain the temperature of the reaction zone at 600.degree.-900.degree. C.
It is known that UF.sub.6 reacts violently with alcohol to form HF, hydrocarbon and UO.sub.2 F.sub.2 or UF.sub.4. This reaction proceeds much faster than the reaction of UF.sub.6 with hydrogen gas.
Further, it is known that this reaction proceeds fast at ordinary temperatures and is exothermic, while the reaction of UF.sub.6 with hydrogen gas proceeds slowly even at 600.degree. C. The present invention is based on these facts as stated above.